1. Field of the Invention
This disclosure relates generally to a method of biaxially orienting thermoplastic materials, and in particular polyethylene terephthalate. In one specific embodiment, a blowable polyethylene terephthalate preform is stretched along its longitudinal axis and then radially expanded to blow air under pressure according to general blow molding techniques.
2. The Prior Art
Various techniques exist in the prior art for both (1) stretching polyethylene terephthalate materials, and (2) blow molding containers of various thermoplastic materials, including polyethylene terephthalate.
For example, U.S. Pat. No. 2,823,421 to Scarlett discloses a method for stretching polyethylene terephthalate film. The method of this patent, however, does not recognize the specific limitations of this invention nor its advantages. In particular, the polyethylene terephthalate film material according to the method of the Scarlett patent is stretched, then reheated, then stretched a second time at a higher temperature, and then heat set at a much higher temperature.
Other methods of treating polyethylene terephthalate material are disclosed in U.S. Pat. Nos. 3,177,277 to Adams and 3,257,489 to Heffelfinger. Again, these patents do not disclose the present invention as will be appreciated more fully from the following detailed description of this invention in comparison to these patent disclosures.
In the specific area of blow molding, U.S. Pat. No. 2,919,462 to Friden is an example of one of the first patents that discloses biaxial orientation of a plastic material, by longitudinally stretching a plastic parison while simultaneously blowing the parison to the configuration of the desired container.
U.S. Pat. No. 3,781,395 to Uhlig discloses and claims a method for sequentially blowing a parison internally of a preform mold, then stretching the blown preform, and then blowing the stretched preform internally of a final blow mold. This patent, however, does not specifically recognize that improved property levels can be achieved in materials that are susceptible to strain-induced crystallization, by effecting a critical major-dimension stretch first and then stretching the material in a second direction to initiate the rapid onset of strain hardening.
Finally, U.S. Pat. No. 3,733,309 to Wyeth discloses a method for blow molding polyethylene terephthalate material, but likewise, fails to recognize and set forth the aspects and advantages of this invention.
In short, the prior art uniformly teaches that property levels are improved by increasing the extent of total stretching or total strain. Accordingly, a problem has heretofore existed in producing extensive strain hardening and strain-induced crystallization at relatively low total areal strains. As used herein, the term "total areal strain" is defined as the quotient obtained by dividing the original surface area into the final surface area of the material after stretching operation.